1. Field of the Invention
This invention pertains generally to semiconductor lasers and photodetectors and, more particularly, to a method for integrating semiconductor lasers and photodetectors on a substrate with a single epitaxial growth and structures that result therefrom.
2. Description of the Background Art
Semiconductor lasers have been used in long-distance (&gt;km) commercial communication systems for over a decade. A well known example is what is referred to as an "in-plane" laser, because the light inside the laser travels parallel to the wafer on which the device is made. More recently, a different laser geometry has become popular for short distance (&lt;100 m) data communications, such as between computers. These lasers are typically known as "vertical cavity surface emitting lasers" (VCSELs). A VCSEL emits light perpendicular to the wafer as the name implies. An advantage of VCSELs is that they are capable of being modulated at high speeds with much lower electrical power than in-plane lasers. In addition, the geometry of VCSELs makes them particularly suitable for making 2-D arrays, and for on-wafer testing. These characteristics can reduce the cost of packaging (which dominates the cost of manufacturing) and costs of the driver circuitry required.
Most VCSELs are "top emitting" devices; that is, light is emitted outward or away from the top surface of the device. However, bottom emitting devices where light is emitted through the substrate are advantageous for systems with arrays of vertical cavity lasers, because the driver circuitry can then be "flip-chip bonded" to the array instead of making individual wire bonds. Arrays of VCSELs may become important in the future to provide even higher speed data transmission links. Such transmission may take place not only between computers, but also within machines, in which case board-to-board and chip-to-chip data communication using VCSELs can be utilized. Processors such as the Intel Pentium II are already limited by the speed of data input and output. For faster data transfer, optical transmission becomes necessary because electrical lines become lossy at high speeds.
A vertical cavity laser comprises a substrate, a bottom mirror, a top mirror and a cavity with a gain medium between the top and bottom mirrors. The gain medium typically comprises quantum wells which, when electrically or optically pumped will emit light. The mirrors typically comprise distributed bragg reflectors (DBRs) formed from alternating high/low index quarter-wave thick layers. Multilayer stacks are generally used for the mirrors instead of metal due to the high reflectivity (&gt;99%) needed to achieve lasing because the gain medium is so thin. Bottom-emitting or top-emitting VCSELs have a partially transmissive bottom or top mirror, respectively. Because of the highly reflectivity mirrors and short cavity used in VCSELs, the lasing wavelength is controlled by the resonant wavelength of the cavity, rather than the peak of the gain as in in-plane lasers. State of the art control of the growth of the cavity can set the wavelength to within .about..+-.0.6% or a few nanometers for lasing wavelength around 1 .mu.m [2]. For a typical bottom emitting vertical cavity laser, the bottom DBR transmission, T.sub.bot, is &lt;1% and the top DBR transmission, T.sub.top, is &lt;&lt;T.sub.bot.
A complete data link typically includes photodetectors as well as lasers. Vertical resonant cavity photodetectors typically comprise a substrate, a bottom DBR mirror, a top DBR or metal mirror, and a cavity with an electrically contacted absorbing medium between the top and bottom mirrors. The mirrors are required in order to be reasonably efficient at converting light into current, and the mirror of lower reflectivity defines the input side of the photodetector. For bottom illuminated photodetectors, where the light input is from the substrate side of the cavity, the wavelength of operation must be transparent to the substrate. In addition, the bottom DBR transmission, T.sub.bot, is typically &gt;5% and the top DBR transmission, T.sub.top, is &lt;T.sub.bot.
Presently the substrates predominately used for VCSELs are GaAs because AlGaAs and GaAs are lattice matched to this substrate and have a relatively large refractive index difference as is needed for making high reflectivity DBR mirrors. However, materials which are lattice matched to GaAs and which absorb/emit at longer wavelengths than GaAs have not been found. Therefore, to obtain absorption/emission at wavelengths longer than GaAs, it is common to use thin, strained layers of InGaAs. By itself, such a thin region would only absorb a few percent of the incident light. In order to obtain a reasonable amount of absorption, mirrors are required to bounce the light many times through the absorbing region. With mirrors, the absorption increases, but at the expense of optical bandwidth. For bandwidths of a few nanometers or more, the input mirror reflectivity must be less than .about.90%, which is much lower than the reflectivity required for a vertical cavity laser.
Packaging of a system of VCSELs and photodetectors, especially in the case of two-way communication, is much simpler and, therefore less costly, if the photodetectors and lasers can be positioned adjacent to each other on the same wafer. Then, connection of the circuits and optical alignment for both can be done at the same time. The extra costs associated with additional processing per device are typically much lower than packaging costs because the additional process steps for integration can be done on a full wafer of several thousand devices all at once whereas extra packaging steps must be done on each device separately. Integration of photodetectors and top-emitting VCSELs has been previously demonstrated [1], and integration is relatively simple. However, there is a need for side-by-side integration of bottom illuminated photodetectors which collect light through the substrate and bottom emitting VCSELs. The present invention quite beneficially achieves such needed integration so that the same epitaxial layer can be used in both devices.